Combustion equipment with catalytic fuel/gas ignition means



June 9, 1964 B. TOONE ETAL 3,136,125

COMBUSTION EQUIPMENT WITH CATALYTIC FUEL/GAS IGNITION MEANS 5 Sheets-Sheet 1 Filed Jan. 50, 1961 June 9, 1964 B. TOONE ETAL 3,136,125

COMBUSTION EQUIPMENT WITH CATALYTIC FUEL/GAS IGNITION MEANS Filed Jan. 30, 1961 3 Sheets-Sheet 2 Fig.5. F' 1 2 .63

June 9, 1964 B. TOONE ETAL 3,136,125

COMBUSTION EQUIPMENT WITH CATALYTIC FUEL/GAS IGNITION MEANS Filed Jan. 50, 1961 3 Sheets-Sheet 3 United States Patent 3,136,125 COMBUSTION EQUIPMENT WITH CATALYTIC FUEL/ GAS IGNITION MEANS Brian Toone, Littleover, Derby, Frank Arkless, Ilkeston,

and Allan Michael Barham, Mickleover, Derby, England, assignors to Rolls-Royce Limited, Derby, England, a British company Filed Jan. 30, 1961, Ser. No. 85,666 Claims priority, application Great Britain July 1, 1960 Claims. (Cl. 60-3932) This, invention relates to combustion equipment, such for example as is used in gasaturbine engines, of the class in which fuel is burnt in a stream of combustionsupporting gas having an elevated temperature and flowing at high velocity.

The invention will be described hereinafter in its application to gas-turbine engines.

According to the invention there is provided a device adapted to promote auto-ignition of fuel injected into the combustion gas stream or to improve stability of combustion or both, which device comprises a mass of refractory material and a foraminate element of platinum, or rhodium, or platinum/rhodium alloy or other suitable metal having catalytic properties in promotion of combustion, which element is supported by the mass which has one or more flow passages extending through it to lead combustion mixture over the foraminate element. The combustion mixture may eventually be led to a flow reversal zone such as is produced in the gas stream by a stabilising gutter.

The refarctory mass may be non-porous, or porous and in the latter case the porous mass may be coated with platinum or rhodium, or an alloy of these, or other suitable catalytic material.

With combustion equipment of the class above referred to there are operating parameters which affect the possibility of securing auto-ignition. For instance, the gas temperature affects ignition and ignition cannot be achieved satisfactorily if the gas temperature is below a particular value which depends upon other operating conditions; also the fuel/air ratio affects auto-ignition and for any particular combustion equipment there is a value of this ratio below which ignition cannot be achieved satisfactorily. Therefore to improve the operational characteristics of the combustion equipment the minimum gas temperature at which auto-ignition of a fuel/air mixture can be obtained, can be reduced or the values of the fuel/air ratio at which weak extinction and ignition occur, can be reduced, or both, or the time required for spontaneous ignition, which time is affected by other operating conditions, can be reduced to as low a value as possible. It is found that if the device of this invention is mounted in a stream of combustion-supporting gas so that there is a flow of the gas through the flow passage or passages in the refractory mass and fuel is injected into the stream at a point upstream of the device, substantial improvement in any or all of these respects can be obtained as compared with a device consisting of a catalysed refractory mass alone, the nature and extent of improvement varying with the formation of the device.

The essential component of the device is the foraminate element of a catalytic metal which gives very short lighting times. The foraminate element must be of very fine construction, and is therefore mechanically weak. Hence it must be supported by a refractory mass and since the mass has a low thermal conductivity, loss of heat from the foraminate element is substantially prevented thus promoting the catalytic reaction. If the refractory mass, when porous, is catalysed by say coating with platinum, the temperature attained by the refracmlC tory mass will be higher and the action of the foraminate element is further improved; in order that the fuel-air mixture shall penetrate to the maximum extent through the porous refractory mass, the pores in the mass must be interconnected to give a high permeability.

Devices which employ a non-porous refractory mass, not only improve ignition, but also are more robust and have a longer life than those employing a porous refractory mass.

To achieve good ignition characteristics, it is necessary to catalyse sul'ficient gas to supply the requisite energy to the fuel-air mixture in the reversal zone of the stabilising gutter. The flow passage or flow passages through the refractory are of a size allowing a sufiicient quantity of the fuel-air mixture to pass directly to the reversal zone, the mixture being catalysed by flowing over the foraminate element.

The refractory mass also lowers the limit at which the flame is extinguished, provided a liquid spray injection is used. This is due to the fact that the gas velocity through passage or passages in the refractory is lower than the general gas velocity. Thus the stream lines flow'roundthe gutter, whereas the liquid droplets having a high momentum carry on towards the refractory mass, which produces a fuel-air ratio in the passages very much higher than the overall fuel/ air ratio. This makes it possible for the foraminate element to maintain the reaction, at much weaker fuel-air ratios than can normally be employed.

When a porous refractory mass is used, only that surface of the device facing the gas flow needs to be porous. The downstream surface is preferably rendered nonporous to reduce flame erosion. The downstream surface and sides may be coated with refractory cement or may consist of a non-porous refractory cup into which the porous refractory mass is cemented. Suitable nonporous refractories are silicon carbide, alumina, and alu minosilicate.

A better catalytic action is obtained with a single large diameter flow passage through the refractory mass than with a number of small passages, but the effect of flame erosion is much greater and it is therefore possible to have either optimum life or optimum catalytic activity. In the case of a large single passage, the upstream portion is normally parallel sided, and downstream portion is either parallel sided or divergent.

The foraminate element may consist of platinum gauze of 0.005 to 0.030" diameter wire, or expanded sheet in which the metal elements are between 0.005-0.030 thick. Platinum alloys containing up to 40% rhodium, iridium, ruthenium or other hardening elements may be used to provide longer life without any loss of catalytic activity. Grain stabilised alloys to prevent loss of strength due to grain growth are also suitable. Rhodium is generally too difficult to form into a foraminate element, but when rhodium is used, it may be in the form of a coating over the whole of the foraminate element and refractory. Rhodium gives slightly improved catalytic activity as compared with platinum. The rhodium may be deposited from a suitable rhodium organic derivation by ignition of the catalyst after soaking in a solution of the organic compound. Alternatively a coating of rhodium may be applied to a platinum foraminate element by normal electro-plating methods.

Other suitable catalytic metals are osmium, and palladium.

Preferably, each device is fitted in a metal support which is welded in position in the stabilising gutter. In order to extend the life of the device, it may be conical or of trapezium section instead of parallel sided, so that gas pressures acting on the device forces it into its metal ruthenium support and the amount of vibration of the device in its holder is reduced.

In jet propulsion gas turbine engines, there may be provided reheat combustion equipment by which turbine exhaust gases are reheated prior to issuing from the engine as a propulsive jet. may be employed with advantage in connection with such reheat combustion equipment, and according to this in-' vention there is provided in such equipment, a combustion stabilising means comprising a channel-seotion stabilising gutter, preferably of V-section, having a series of devices as above set forth spaced apart along the channel section member and projectiing through its base so as to have one end of the flow passages externally of the member andthe other end' of the flow passages open to the channel. For instance 9 devices may be disposed around annular gutter. Alternatively if two or more annular gutters are used the catalysts may be fitted in interconnecting gutters which spread the flame from one gutter to another. In this case three devices may be found to be sufiicient.

. Some forms of device according to this invention will now be described with reference to the drawings in which:

FIGURE 1 is an axial section through one form of device,

' FIGURE 2 is a view in the direction of arrow 2 on FIGURE 1 and shows the section line 1-1 of FIGURE 1 FIGURE 3 is a view partly in section of gas turbine reheat combustion equipment, 7

FIGURES 4 and 4A show another form of device, FIGURE 4' being a section on the line 4-4 of FIGURE 4A, and V FIGURES 5, 5A and 6, 6A and 7, 7A and 8, 8A and 9, 9A and 10, 10A are view corresponding to FIGURES 4 and 4A of further forms of device,

FIGURE 11 shows yet another form of device according to this invention in axial section,

FIGURES 11A, 11B, 11C, 11D show the individual parts of the device of FIGURE 11, FIGURE 113 being a view in the direction of arrow B on FIGURE 11A,

FIGURE 12 shows a modified form of foraminate element,

FIGURE 13 shows another modified form of foraminate element,

FIGURE 14 shows in axial section another form of device according to this invention,

FIGURES 14A, 14B, 14C, 14D show the individual parts of the device of FIGURE 14, FIGURE 140 being a view in the direction of arrow C on FIGURE 14B, and

FIGURE 15 shows a modification of the devices of FIGURES l1 and 14.

The device (FIGURES 1 and 2) comprises a mass of catalysed, porous high permeability, refractory material, for instance a high strength fire brick which has been soaked in a 10% solution of chloroplatinic acid, allowed to dry and then ignited at 850 C., the treatment being repeated if desired. The mass is in the form of two cylindrical ceramic plugs III, 11 each have a central axial bore 10a, 11a of about A inch diameter. Other suitable refractory materials would be permeable alumina, silica, alumina silicate, or other refractory oxides.

The devicealso comprises a foraminate element 12, of platinum, rhodium, or platinum/rhodium allow which is a fiat piece, for example a disc, of wire gauze or expanded metal, sandwiched between the two plugs 10, 11 and extending across the bore 10a, 11a.

The device also comprises a non-porous cup-shaped casing 13 of ceramic material in which the sandwich 10, 11, 12 is cemented. The casing 13 has a base in the form of an inwardly-directed flange 14, the inner edge of which defines an aperture aligned with and of the same diameter as the bores 10a, 11a. The external surface of the flange 14 has a frusto-conical recess 15 diverging away from the aperture.

The devices of this invention a The bores 10a, 11a have a very thin coating 10b of dispersed platinum as a result of the soaking and firing process described above and form a flow passage running through the device to conduct fuel/ air mixture to the element 12.

t One use of such a device is illustrated in FIGURE 3 which shows diagrammatically reheat combustion equipment of a gas-turbine jet propulsion engine. The engine has a jet pipe 16 conveying engine exhaust gases to a propulsion nozzle 17. The reheat combustion equipment comprises a ring of liquid fuel injectors 18 supplied from a manifold I? by branches 19a and a combustion stabiliser 20 disposed downstream of the injectors 18. It is important that the fuel shall have lost most of its injection momentum when it reaches the catalyst, or ignition will be adversely affected. Thus the injectors preferably inject the fuel upstream, not directly downstream on to the catalyst.

The combustion stabiliser 20 comprises a V-section channel member 21 of annular form having a mean diameter equal to the diameter of the circle on which the injectors 18 lie, the channel 21a facing downstream, and a series of devices 22, such as described above or hereinafter, disposed in equiaangular relation around the member 21, there being for instance one in line with each injector 18. When the devices 22 are as described with reference to FIGURES 1 and 2, they are fitted in the member 21 with part projecting upstream of the member 21 and with the flange 14 forming the downstream surface, stream.

The stabiliser 20 is supported by struts 23.

In use, a mixture of fuel and exhaust gas, which has an elevated temperature and contains a large proportion of unburnt oxygen, flows downstream from the region of the injectors 18 and a small flow of the mixture passes through the bores 10a, 11a into the reversal zone afforded by the channel 21:: and in so doing passes over the foraminate element. Also part of the mixture enters the pores of the porous refractory mass. The combined effect of the catalysed mass and of the foraminate element is to cause ignition of the mixture and the flame is propagated through the reversal Zone to the main body of mixture flowing in the jet pipe.

The ignition time and the porous surfaces of the devices facing upthe temperature of the mixture below which spontaneous ignition is not achieved, varies with the construction of the devices 22 and with the fuel/air ratio. With a device such as is shown in FIG- URES 1 and 2, ignition times below 5 see. are obtainable over wide ranges of mixture temperatures and of fuel/air ratio and weak extinction occurs only at very low fuel/ air ratios.

In another form of, catalytic device, FIGURES 4 and 4A, the foraminate element 30 is gauze or expanded sheet and is mounted on the downstream face of a catalysed porous refractory plug 31 having a series of passages in the form of bores 32 extending through it from its upstream face 31a to its downstream face, all of the bores leading to the foraminate element 30 so that the fuel/ air mixture flowing through them passes through the element 31] before entering a reversal zone behind a stabilising gutter. This form of device enables ignition to be achieved at relatively low temperatures and reduces markedly the value of the fuel/ air ratio at which weak extinction occurs.

In FIGURES 5 and 5A, the foraminate element 34 is again a disc or gauze or expanded metal and it is sandwiched between two plugs 35, 36 of catalysed porous refractory material, the plugs having aligned axial bores 35a, 36a forming the flow passage 'or passages. In this form, the fuel/air mixture passes through the foraminate element in flowing from the bores 35a in the plug 35 to the bores 36a in the other plug 36. The use of this form of device results in markedly improved ignition and in a reduction of the weak extinction values at least at some gas temperatures.

In FIGURES 6 and 6A, the refractory mass comprises two tubular members 37, 38, each having a single bore 37a, 38a running axially through it. A disc 39 of gauze or expanded catalytic metal is located between the members 37, 38. The bore 38a which is on the downstream side of the disc 39, diverges in a direction away from the disc 39. Markedly improved ignition characteristics are obtained with this form of device whether, as is preferred, the divergent portion of the bore of the tubular member is downstream of the disc, or upstream of it.

In the form of device shown in FIGURES 7 and 7A, which is superior mechanically to the other forms of device above described, the foraminate element is a roll or cylinder 40 of gauze or expanded catalytic metal housed in a transverse bore 41 in the refractory mass 42 which has a flow passage 43, 44, extending through it and intersecting the transverse bore 41. The portion 43 of the flow passage to one side of the transverse bore 41' is cylindrical and the portion 44 to the other side is divergent in a direction away from the transverse bore 41. The smaller diameter of the portion 44 may be of larger diameter than that of the cylindrical portion 43. This form of device also gives markedly improved ignition characteristics whether the device is disposed in the gas stream with its divergent flow passage portion facing downstream, as is preferred, or facing upstream.

The forms of device shown in FIGURES 4 to 7A are preferably assembled within a non-porous refractory casing, for example as shown in the device of FIGURE 1, to give the device additional strength and resistance to erosion over its downstream surface.

In the form of device shown in FIGURES 8 and 8A, plug 45 of porous refractory material has 4 to 6 bores or passages 46 which lead from the upstream surface 45a to a foraminate plate 47 on the downstream surface of the plug. Theplug 45 is fitted into a non-porous cup shaped case 48, the downstream face of which has a M1." exit hole 49. The inner surface of the case has a recess 50 which converges towards the hole 49 to form a collecting zone for the fuel/ gas mixture flowing through the bores 46.

In the forms of device shown in FIGURES 9, 9A and l0 and A the refractory mass 51 is of trapezium section in the direction of gas flow and in a segment of an annulus. A series of such segments are fitted in or form the base of a stabilising gutter.

In FIGURES 9 and 9A, the foraminate element 53 is a flat piece of gauze or expanded metal embedded in the mass, there being flow passages 52 extending from the narrower end of the mass past the element 53 to the wider end which will be disposed to face downstream. In FIGURES l0 and 10A, the foraminate element 54 is a coil or cylinder of gauze or expanded catalyticmetal housed in a transverse bore 55 intersecting the passages 52.

The refractory mass 51 has grooves 56 in it to enable it to be held in place by corresponding ribs formed by externally grooving the sides of a V-section stabilising gutter.

' The minimum, diameter of the flow passages through the refractory mass is determinedby trial and is selected so that (a) there is a suflicient flow to enable catalysis to take place, (b) there is a sufiicient flow of air/fuel mixture to the foraminate element for ignition to occur and for the flame to propagate to the reversal zone behind the stabilising gutter, and (c) the flow is not so high as to materially raise the weak extinction limit. For multi-passage' devices, a passage diameter of 0.060" to 0.150 has been found suitable. The preferred diameter where only a single hole is used is approximately 0.250".

As suitable porous refractory materials, GR-28 firebrick or other alnmino-silicates may be mentioned. The simpler types of catalyst which contain a horizontal roll or plate of foraminate show poor catalytic activity if the foraminate is thicker than 0.010". Activity increases as the thickness decreases. The life of the catalyst on the other hand increases with thickness. Thus it may be found necessary to use thicker foraminate.

In modifications of the above described devices, the passages 10a; 32; 35a, 36a; 37a, 38a; 43, 44; 46; 52 may be lined with perforated metal sheet, or expanded metal, or wire gauze. In the case of a plug as shown in FIG- URE l for example, 10b may represent such a lining.

This modification improves the activity of the catalyst. It may then be considered desirable to use a thicker foraminate material say 0.020. The improvement is then lost, but there is a final gain in mechanical strength and hence usable life.

Further modifications of the foraminate element are as follows. The passages 10a, 32, 35a, 36a, 37a, 38a, 43, 44, 46, 52 may be lined with two or more rolls of perforated metal sheet, or expanded metal or wire gauze 63. It is also possible to use a helix for one or more of the rolls. The transverse roll may also be modified in this way.

These modifications are more expensive since the amount of platinum is increased, but they enable a good catalytic activity to be obtained with a non-porous refractory. This again results in much better resistance to hot erosion inside the bores.

The form of device shown in FIGURE 11 to 11D comprises a cylindrical mass 60 (FIGURES 11 and 11D) of non-porous refractory material, for instance alumina or silicon carbide, or silicon nitride, or an alumino-silicate which is a good heat insulator, has good thermal shock properties and does not attack a platinum-rhodium foraminate material at high temperatures. It may be necessary to coat or flame spray silicon carbide and silicon nitride with aluminum or zirconia to prevent free silicon contaminating the catalytic alloy. The mass 60 has a central bore 60a, which may be 0.25" in diameter.

The device also comprises a foraminate element of perforated or expanded sheet metal or of wire gauze, a suitable metal being platinum/ rhodium alloy, which may take any of the forms shown in FIGURES 11A, 11B, or in FIGURE 12 or in FIGURE 13. The material from which the foraminate element is made may be up to 0.030 inch thick.

In FIGURE 11A, the foraminate element comprises a disc portion 61 which when the device is assembled lies against one face of the cylindrical mass (FIGURE 11) and a cylindrical portion 62 which occupies the bore 60a, the portions 61, 62 being welded together.

In FIGURE 12, the foraminate element has a second cylindrical portion 63 of smaller diameter than the portion 62 arranged coaxially within the latter and welded by one end to the disc portion 61.

In FIGURE 13 the foraminate element has a helix 64 of platinum/ rhodium wire welded to the internal surface of the cylindrical portion 62.

The device also includes a cup-shaped case 65, the base 65a of which has a small hole 66 centrally in it. The case 65 is made of a refractory material having a substantially higher strength than the refractory material forming the mass 60, and is for instance made from alumina alumina-silicate, silicon carbide, silicon nitride or the material marketed under the trade name Refrax.

The parts of the device are assembled with the disc porof the foraminate element and as to the shape of the mass 60 which is modified to accommodate the dilferent form of the foraminate element. The foraminate element is T-shaped and comprises a cylindrical portion 67 to occupy the bore 60a in the mass 60, and a transversely disposed cylindrical portion 68 which occupies a recess 6% in the end of the mass 60 which lies against the base 65a of the case 65. The cylindrical portion 67 may have a second and coaxial cylindrical portion within it as shown.

in the form of foraminate element of FIGURE 12 or may have a wire helix secured to its internal surface like the foraminate element of FIGURE 13. The same modifications may be applied to the transversely disposed cylindrical portion 63.

The forms of device described with reference to FIG- URES 11 to 14 are robust and have a long life when used in gas turbine reheat combustion equipment, for example as described with reference toFIGURE 3, and they give low ignition times over a wide range of gas flows and over a wide range of fuel/air ratios.

The ranges of fuel/ air ratios and gas flows over which the devices operate satisfactorily may be improved by restricting the'entry to the bore 6%. This is particularly effective if the fuel injectors are within 20 inches of the catalysts. For example as shown in FIGURE 15, the device may have a disc 69 bonded to that surface facing upstream in the gas flow and having a central hole 69a giving access to the bore 60a, the hole 69a having a di: ameter selected to be substantially smaller than that of the bore 60a, and may vary from 0.0l0.240 inch. The restriction of flow in this way not only reduces the pos sibility of the occurrence within the bore 60a of high concentrations of liquid fuel tending to cool the foraminate element to such an extent that ignition is prevented, but also promotes reversal flows within the bore so improving contact between the foraminate element and the fuel/air mixture and making the device less sensitive to changes in rate of flow of the main stream. This type of restriction can only be used with the more complex foraminate device, be porous.

We claim:

1. A prime mover ignition device adapted to promote ignition of fuel injected into a stream of combustion-sup-f porting gas comprising a mass of refractory material having upstream and downstream facing surfaces, at least one bore extending through the mass from said upstream surface to the downstream surface and forming a flow passage for the combustion-supporting gas and fuel in admixture, and a foraminate element of platinum, or

rhodium, or platinum/rhodium alloy or other metal having catalytic properties in promotion of combustion, which element is less than 0.030 inch thick and is supported by the mass, said flow passage, leading the combustion mixture to the foraminate element.

2. A device according to claim 1, wherein the refractory mass is porous and is itself catalysed.

3. A device according to claim 1, wherein the foraminate element comprises a foraminous flat piece of the catalytic metal extending across the flow passage.

4. A device according to claim 1, wherein the foraminate element comprises a foraminous flat piece of the catalytic metal-extending across the flow passage and a cylinder of a foraminous catalytic metal lining the flow passage. 7

5. A device according to claim 4, wherein the cylinder lining the flow passage has locatedwithin it at least one substantially smaller diameter cylindrical portion of the foraminous catalytic metal.

6. A device according to claim 4, comprising a helix in which the ceramic plugs need not of Wire of the catalytic metal welded to the inner surface of the said cylinder lining the flow passage.

7. A device according to claim 1, the bore forming'the flow passage being parallel sided upstream of the foraminate element and being divergent downstream thereof.

8. A device according to claim 1, comprising a cupshaped case accommodating the refractory mass and foraminate element, the cup-shaped case being of nonporous refractory material and, the case having a base with an aperture therein aligned with the fiow passage, the base covering the downstream surface of the refractory mass.

9. A device according to claim 8, the refractory mass having a plurality of parallel bores, each forming a flow passage, the base of the cup-shaped case having therein a recess which faces the refractory mass and converges towards the aperture to form a collecting zone for the fuel/gas mixture flowing from the passages.

10. In or for combustion equipment of the class wherein fuel is burnt in a stream of combustion-supporting gas, the combination with a stabilizing gutter for producing a flow reversal zone in the stream, of at least one device according to claim I mounted in the gutter so that there is a how of fuel/gas mixture through each flow passage to the foraminate element and then into the reversal zone.

11. A device according to claim 1, wherein the foraminate element comprises a roll of the foraminous catalytic metal extending across the flow passage.

12. A device according to claim 11, said mass having a transverse bore intersecting said flow passage, the roll being received in the transverse bore and extending across the fiow passage between the ends thereof.

13. A device according to claim 11, thefractory mass havinga transverserecess extending across the downstream face of therefractory mass and intersecting the flow passage, the roll being in the recess and extending across the flow passage.

' 14. A device adapted to promote ignition of fuel injected into a stream of combustion-supporting gas comprising a mass of refractory material, said mass having an upstream surface and a downstream surface and said mass having at least one bore extending therethrough from said upstream surface to the downstream surface, which bore affords a flow passage for a fuel/combustion-supporting gas mixture and which borehas a diameter in the rangefrom 0.010 inch to 0.25 inch, and a foraminate catalytic element of a metal selected from the group consisting of platinum, rhodium and alloys of platinum and rhodium carried by said mass, said catalytic element having a thickness of up to .030 inch, said bore leading the the mixture into contact with the foraminate element.

15. A device according to claim 1 comprising means providing a restricted entry to the flow passage and creating a flow reversal within the passage, said means comprising a disc secured to the upstream face of the refractory mass and having therein an inlet hole of smaller diameter than the passage.

References Cited in the file of this patent Great Britain Sept. 9, 1953 

1. A PRIME MOVER IGNITION DEVICE ADAPTED TO PROMOTE IGNITION OF FUEL INJECTED INTO A STREAM OF COMBUSTION-SUPPORTING GAS COMPRISING A MASS OF REFRACTORY MATERIAL HAVING UPSTREAM AND DOWNSTREAM FACING SURFACES, AT LEAST ONE BORE EXTENDING THROUGH THE MASS FROM SAID UPSTREAM SURFACE TO THE DOWNSTREAM SURFACE AND FORMING A FLOW PASSAGE FOR THE COMBUSTION-SUPPORTING GAS AND FUEL IN ADMIXTURE, AND A FORAMINATE ELEMENT OF PLATINUM, OR RHODIUM, OR PLATINUM/RHODIUM ALLOY OR OTHER METAL HAVING CATALYTIC PROPERTIES IN PROMOTION OF COMBUSTION, WHICH ELEMENT IS LESS THAN 0.030 INCH THICK AND IS SUPPORTED BY THE MASS, SAID FLOW PASSAGE, LEADING THE COMBUSTION MIXTURE TOTHE FORAMINATE ELEMENT. 